The present invention relates to a device for supplying radiant energy onto a printing substrate comprising at least one radiant energy source whose light impinges on the printing substrate on the path of the printing substrate through a printing press at a position downstream of at least one printing nip in a printing unit.
Depending on the type of printing ink and the underlying particular drying process, printing machines, in particular, planographic printing presses such as lithographic printing presses, rotary printing presses, offset printing presses and the like, which process sheet or web stock, in particular, paper, cardboard, carton and the like, are known to have different devices which initiate or promote adhesion of the ink to the printing substrate by supplying radiant energy to the printing ink present on the substrate.
The so-called “UV inks” cure by polymerization, which is triggered by photoinitiation using ultraviolet light. On the other hand, solvent-containing printing inks, which are able to undergo both a physical as well as a chemical drying process, are very common. The physical drying includes the evaporation of solvents and the diffusion into the printing substrate (absorption), whereas “chemical drying” or “oxidative drying” are understood to mean drying due to the polymerization of the oils, resins, binding agents or the like included in the ink formulations, possibly with the participation of atmospheric oxygen. Generally, the drying processes are dependent on each other since, because of the absorption of the solvents, a separation between solvents and resins occurs within the binder system, as a result of which the resin molecules can get close to one another and, possibly, be polymerized more easily.
It is already known to subject printed products to a drying process after the printing process to allow the printed products to be further processed without delay. At this point, mention should be made, for example, of UV inks in conjunction with UV driers, heatset inks in conjunction with hot-air dryers or IR dryers.
However, UV inks are considered critical to health and need to be disposed of separately. Moreover, the UV radiation creates ozone, resulting in the requirement for expensive exhaust equipment or inerting procedures.
In contrast, heatset drying requires a large amount of energy and may lead to excessive drying of the printing substrate and, thus, to unwanted warping.
The use of spectrally broadband IR dryers can also lead to excessive drying and, thus, to unwanted warping of the printing substrate, because the larger portion of the energy is absorbed by the printing substrate while only a small portion is absorbed by the actual printing ink to be dried.
Moreover, the use of drying accelerators, so-called “siccatives”, in the printing ink can easily lead to premature drying of the printing ink and, thus, to build-up of printing ink on the surfaces of the printing unit cylinders. Therefore, the dosage of siccatives is limited.
German Patent Application No. 102 34 076 A1, for example, describes a device for drying printing ink on printing substrates, including a radiant energy source, in particular a laser, which emits light in the near infrared region. The wavelength of this IR radiation is selected to be non-resonant to absorption wavelengths of water, which makes it possible to heat only the ink but not the sheet.
Moreover, for example, German Patent Application No. 103 16 471 describes a method for drying a printing ink on a printing substrate, in which the printing substrate is exposed to laser radiation whose wavelength is between 350 nm and 700 nm, and is substantially resonant to an absorption wavelength of at least one color pigment of the printing ink. Besides the pigment, no other absorbing agent is needed for the radiation.
Furthermore, for example, German Patent Application No. 103 16 472 discloses a method for drying a printing ink on a printing substrate, in which, in addition to the printing ink, a primer or coating is applied to the printing substrate; the primer or coating being suitable to accelerate the drying of the printing ink by absorption of radiation.
European Patent Application No. 0 355 473 A2, for example, describes a device for drying printed products, which includes a radiant energy source in the form of a laser. The radiant energy is transmitted to the surface of the printing substrates, which are conveyed along a path through the printing press by a transport device, at a position between individual printing units or following the last printing unit, before or in the delivery. In this context, the radiation source can be a laser in the ultraviolet for UV inks or a laser in the infrared for heating solvent-containing printing inks. The radiant energy source is located outside the printing press to prevent parts of the press from being undersirably heated because of dissipation heat that cannot be avoided or shielded. Here, the disadvantage is, however, that an additional system component must be separately provided to the user of the printing press.
Moreover, it is known, for example, from U.S. Pat. No. 6,026,748 that a printing press can be provided with a dryer system featuring infrared lamps which emit short-wave infrared light (near infrared) or medium-wave infrared light. Lamp light sources have a wide-band emission spectrum, offering a multitude of wavelengths. The drawback of such drying devices in the infrared region is that a considerable proportion of the energy absorption takes place in the paper, the ink only being indirectly heated. A rapid drying is only possible by inputting enough energy. In the process, however, there is the danger, inter alia, of the printing substrate drying out unevenly and becoming warped.
In electrophotographic printing, it is known, for example, from German Patent Application No. DE 44 35 077 A1, to fix toner on a recording medium using radiant energy in the near infrared region emitted by diode lasers. A narrow-band light source is used to heat the toner particles, in order to melt them, to form them into a colored coating, and to anchor them to the surface of the recording medium. Since a great number of common paper types have broad absorption minima in this spectral range, it is possible for a predominant part of the energy to be directly absorbed in the toner particles.
However, the simple knowledge of the window in the paper's absorption spectrum cannot be directly exploited in printing technology that uses solvent-containing printing inks, since, as described above, there are other underlying chemical and/or physical drying processes.